You need a wireless router, and some type of wireless device for the computers to communicate with the router.
Wireless is slower than wired, simply because it travels over air- although you won't notice much of a difference.
10Mbps/100Mbps/1000Mbps is how fast the transfer of data is. 10Mbps is 10 mega-bits per second, 100Mbps is 100 mega-bits per second, and 1000Mbs is actually 1 giga-byte per second. The higher the number, the faster the transfer. Keep in mind that although many devices claim they can handle this type of data transfer, most ISP (Intenet Service Providers) only have up to 1-8Mbps download speeds. Some have more, but realistically, you will see on a good day about 3-6Mbps on the fastest connection- at least in the Tennessee area. 802.11g is a wireless networking standard. You can Google that number, and you will get many links to detail the specification for you. All you have to do is have both computers on the same workgroup, with different names, and share the printer. This works best if both PCs are Windows XP, but it is possible to mix and match- just harder.
The difference between a PCI wireless card and a USB wireless card is just the way they plug into your computer. A PCI wireless card goes inside your PC, in a PCI slot. A USB wireless card plugs into a USB port on your computer. In my opinion, the USB are better, I've seen too many issues with the PCI ones.